Add RobloxString to rbx_types, use it for instance names

[kennethloeffler]

This PR partially addresses #475 by adding a new RobloxString enum to rbx_types and using it for InstanceBuilder.name and Instance.name.

This PR changes nearly every test snapshot we have, so the diff is crazy, I'm sorry.

There's still some stuff left to do here:

• Add documentation to RobloxString
• Add Deserialize implementation to RobloxString
• Review RobloxString's API - maybe there should be a to_utf8_lossy method or something like it?
• Maybe there should be warnings when we need to lossily serialize a RobloxString::Binary?

[kennethloeffler]

I'm not really sure if we want to merge this in for the next batch of releases. It is a breaking change to rbx_dom_weak, and we've already made some breaking changes so there is some merit to including it, but there's still a decent amount of work and review left on it and it's probably not worth blocking the big performance improvements we have in the pipeline.